There is significant interest in low cost flexible electronic displays. Typically, such displays comprise a light modulating component embedded in a binder, most commonly polymer, matrix that is coated over a conductive plastic support. Broadly speaking, a light modulating component is a material that changes its optical properties such as its ability to reflect or transmit light in response to an electric field. The light modulating component may be a liquid crystalline material such as a nematic liquid crystal, a chiral nematic or cholesteric liquid crystal or a ferroelectric liquid crystal. The light modulating material may also be a water insoluble liquid containing particles that undergo electrophoresis or motion such as rotation or translation in response to an electric field. Displays comprising a liquid crystalline material in a polymer matrix are referred to as polymer dispersed liquid crystal (PDLC) displays.
There are two main methods for fabricating PDLC devices: emulsion methods and phase separation methods. Emulsion methods have been described in U.S. Pat. Nos. 4,435,047 and 5,363,482. The liquid crystal is mixed with an aqueous solution containing polymer. The liquid crystal is insoluble in the continuous phase and an oil-in-water emulsion is formed when the composition is passed through a suitable shearing device, such as a homogenizer. The emulsion is coated on a conductive surface and the water allowed to evaporate. A second conductive surface may then be placed on top of the emulsion or imaging layer by lamination, vacuum deposition, or screen printing to form a device. While the emulsion methods are straightforward to implement, droplet size distributions tend to be broad resulting in a loss in performance. For cholesteric liquid crystal devices, also referred to herein as CLC devices, this typically means reduced contrast and brightness. Phase separation methods were introduced in an effort to overcome this difficulty.
Phase separation methods have been outlined in U.S. Pat. No. 4,688,900 and in Drzaic, P. S. in Liquid Crystal Dispersions, pgs. 30-51, published by World Scientific, Singapore (1995). The liquid crystal and polymer, or precursor to the polymer, are dissolved in a common organic solvent. The composition is then coated on a conductive surface and induced to phase separate by application of ultraviolet (UV) radiation or by the application of heat or by evaporation of the solvent, resulting in droplets of liquid crystal in a solid polymer matrix. A device may then be constructed utilizing this composition. Although phase separation methods produce dispersed droplets having more uniform size distributions, there are numerous problems with this approach. For example, the long term photostability of photopolymerized systems is a concern due to the presence of photoinitiators that produce reactive free radicals. Photoinitiators not consumed by the polymerization process can continue to produce free radicals that can degrade the polymer and liquid crystals over time. Furthermore, it is also known that UV radiation is harmful to liquid crystals. Specifically, exposure to UV radiation can lead to decomposition of the chiral dopant in a cholesteric liquid crystal mixture, resulting in a change in the reflected color. The use of organic solvents may also be objectionable in certain manufacturing environments.
U.S. Pat. Nos. 6,423,368 and 6,704,073 propose to overcome the problems associated with the prior art methods through the use of droplets of the liquid crystal material prepared using a limited coalescence process. In this process, the droplet-water interface is stabilized by particulate species, such as colloidal silica. Surface stabilization by particulate species such as colloidal silica is particularly preferred as it can give narrow size distribution and the size of the droplets can be controlled by the concentration of the particulate species employed. The materials prepared via this process are also referred to as Pickering Emulsions and are described more fully by Whitesides and Ross (J. Colloid Interface Sci. 169, 48 (1995)). The uniform droplets may be combined with a suitable binder and coated on a conductive surface to prepare a device. The process provides improvement in brightness and contrast over prior art processes. It also overcomes some of the problems associated with photoinitators and UV radiation. However, there is still much room for improvement, particularly in terms of the switching voltage or the voltage needed to change the orientation of the liquid crystal from one state to another. The latter has a significant effect on the overall cost of the display. A low switching voltage is extremely desirable for low cost displays.
The device described by U.S. Pat. Nos. 6,423,368 and 6,704,073 suffers from drawbacks because of the structure of the coated layer. Undesirably, there may be more than a monolayer of droplets between the two electrodes. Furthermore, the process of coating a heated emulsion of the liquid crystal in a gelatin binder onto a substrate with a conductive layer and subsequently lowering the temperature of the coating to change the state of the coated layer from a free flowing liquid to a gel state, referred to as a sol-gel transition, prior to drying the coating results in an extremely uneven distribution of droplets of liquid crystal. At the microscopic scale there are regions of the coating containing overlapping droplets and other regions with no droplets at all between the electrodes. The uneven distribution of droplets results in a decrease in contrast and an increase in switching voltage.
U.S. Pat. Nos. 6,271,898 and 5,835,174 also describe compositions suitable for flexible display applications that employ very uniform sized droplets of liquid crystal in a polymer binder. However, no attempt is made to control the thickness or the distribution of droplets in the coated layer resulting in less than optimum performance.
U.S. patent application Ser. No. 10/718,900 shows that the maximum contrast in a bistable chiral nematic liquid crystal display prepared by the limited coalescence method is obtained when the uniform liquid crystal domains or droplets are coated as substantially a monolayer on the first conductive support. The bistable states in these chiral nematic liquid crystal displays are the planar reflecting state and the weakly scattering focal conic state. Back-scattering of light from the weakly scattering focal conic state increases drastically when there is more than a monolayer of droplets between the conductive surfaces. While the method provides displays with an improvement in brightness and contrast, it still falls short of optimum performance because the gelatin binder is made to undergo a sol-gel transition prior to drying of the coating resulting in an uneven structure.
Rudhardt et al. (Applied Physics Letters vol. 82, page 2610, 2003) describe a method of fabricating a light modulating device wherein a composition containing very uniform droplets of liquid crystal in an aqueous solution of polymer binder is spread on an indium tin oxide (ITO) coated glass surface and the water allowed to evaporate. The droplets of liquid crystal spontaneously self-assemble into a hexagonal close-packed (HCP) monolayer. A second ITO coated glass surface is placed over the coated layer of droplets as the top electrode to complete construction of the device. A uniform monolayer thickness is achieved for the coated layer and the close-packed distribution of droplets is also extremely well defined. Both features result in a low switching voltage. However, there are numerous problems with this approach. Firstly, the uniform droplets of liquid crystal are prepared by extrusion through a thin capillary into a flowing fluid. When a droplet at the tip of the capillary grows to reach critical size, viscous drag exceeds surface tension and breakoff occurs, producing highly monodisperse emulsions. Clearly, this method of creating one droplet at a time is not suitable for large scale manufacture. Secondly, the method by which the second, that is, top, electrode is applied may be suitable for construction of small scale displays on rigid substrates such as glass but is not viable for large area low cost displays on flexible substrates.
US 2003/0137717A1 and US 2004/0217929A1 indicate that a close-packed monolayer of droplets of the light modulating component may be desirable for obtaining high brightness and contrast in a polymer dispersed electrophoretic display. However the method of making droplets described in these applications is a standard emulsification process that does not result in emulsions having a narrow size distribution that is desirable for obtaining close-packed monolayers by spontaneous self-assembly. The preferred method of preparing droplets in US 2003/0137717A1 and US 2004/0217929A1 also involves encapsulation resulting in droplets or capsules in the size range of 20 to 200 microns with wall thickness of 0.2 to 10 microns. The relatively large droplet size and wall thickness result in high switching voltages. The latter is particularly a problem for bistable CLC devices. Encapsulation is clearly not desirable but these applications do not teach how a second conducting layer is to be applied on top of the coated layer of droplets in the absence of encapsulation. In the absence of encapsulation, droplets of the light modulating component may directly come in contact with the organic solvent in the screen printed conducting ink leading to contamination or poisoning of the light modulating component. This is particularly a concern if the light modulating component is a liquid crystal material.
To overcome the difficulties of US 2003/0137717A1 and US 2004/0217929A1, US 2004/0226820A1 teaches that a close-packed monolayer of droplets may be obtained by using electro-deposition followed by washing after the droplets have been spread on a suitable surface using a coating knife or coating head such as a slot die coating head. However, the additional steps of electro-deposition and washing are cumbersome and not suitable for manufacturing on a large scale. Even with these additional steps, a close-packed monolayer of uniform thickness is not achieved. The root mean square (RMS) surface roughness is about 6 microns because of non-uniform droplets or capsules. This is a very high value of surface roughness that would result in irregular or incomplete curing if a UV curable screen printed conducive ink is used as the second electrode. The irregular curing will result in increased switching voltages. Furthermore, a surface roughness of this magnitude will also result in significant non-uniformity of switching voltage across the area of the display since the switching voltage is directly related to the thickness of the coated layer.
US 2003/0137717A1, US 2004/0217929A1 and US 2004/0226820A1 also teach using polymer latex as the preferred binder. The use of polymer latex materials is not desirable for a number of reasons. Many commercial latex materials contain high boiling organic co-solvents that render them unsuitable for use in PDLC films due to the poisoning effect the solvents have on the liquid crystal or other light modulating component. This is particularly true if the droplets are not encapsulated as is desirable from the point of view of reduced switching voltage. Latex polymers also have an affinity for the liquid crystal or other light modulating component leading to dissolution of the light modulating component into the polymer matrix. Furthermore, if the latex is not fully transparent, it can lead to a loss of contrast. Other binders suggested in US 2004/0217929A1 such as acrylics or polyvinylalcohol are difficult to fix or cross-link if used alone. Fixing or cross-linking is desired in order to preserve the close-packed monolayer structure when other layers are spread over it.
U.S. patent application Ser. No. 11/017,181 describes a novel method for polymer dispersed liquid crystal that overcomes the problems of the prior art. A uniform dispersion of liquid crystal droplets is prepared by the limited coalescence process. The droplets are mixed with a suitable binder and coated and dried on a flexible conductive support at a temperature above the sol-gel transition of the binder. The uniform droplets of liquid crystal spontaneously self-assemble to create a close-packed monolayer. The desired close-packed structure is then fixed or preserved by cross-linking the binder. Subsequently, a second aqueous layer containing gelatin is coated above the liquid crystal layer and allowed to dry at a temperature that is below the sol-gel transition of the binder. This second layer protects the liquid crystal material from the solvent in the conductive ink. A conductive ink is screen printed over this layer to complete construction of the device. The device may be manufactured using a low-cost process. Furthermore, it exhibits low switching voltage as well as good contrast and brightness. However, there is still room for further reductions in switching voltage.
To create a device with low switching voltage, it is important to minimize the voltage drop across the second aqueous layer or barrier layer. While it is possible to do this by using conductive materials such as dispersed carbon black, use of such isotropic conductors results in undesirable field spreading. In a matrix display with multiple rows and columns, it is important to activate only the desired pixel and not adjacent pixels. To enable this, it is essential to avoid electrical conduction in the x-y plane, that is, parallel to the electrodes, in the barrier layer but have significant electrical conduction in the z-direction, that is, perpendicular to the electrodes.
An alternative approach is clearly needed that improves upon the device architecture of U.S. patent application Ser. No. 11/017,181 to achieve low switching voltages.